An important trend in modern semiconductor technology is the design and fabrication of an integrated circuit (IC) on a semiconductor substrate that includes not only components processing digital signals, e.g., programmable logic devices (PLD), but also components that manipulate analog signals, e.g., capacitors and inductors. Such a mixed-signal type of IC device is particularly useful in wireless applications. For example, a key component of a digital cellular phone is a radio frequency (RF) tuned circuit that can pass or reject analog signals at a certain frequency band. If the tuned circuit is integrated with other components of the phone on a substrate surface, it may significantly reduce the size of the phone as well as its noise level and power consumption.
A crucial component in the tuned circuit is a LC circuit comprising at least one inductor and one capacitor, whose self-resonant frequency determines the frequency range over which the cellular phone can be operated. The LC circuit's self-resonant frequency increases with its inductor's inductance. Since inductance increases with physical size of the inductor, a higher self-resonant frequency requires a physically larger inductor. To attain a required inductance for certain applications, the inductor can be of significant physical size and if implemented in a semiconductor device, requires a large surface area of the semiconductor substrate. To offset this requirement for large physical size, the inductor is typically formed on the surface of a substrate in a spiral form in two dimensions.
FIG. 1 is 2-D a plain view of a prior art horizontal spiral inductor fabricated on the surface of a semiconductor substrate 100. The spiral inductor includes four turns and two input/output terminals 120 and 140, all formed in the same layer of metallization on top of a semiconductor substrate. Since all four legs of each turn of the spiral inductor are located on the same horizontal surface close to the substrate, this may cause a serious parasitic capacitance between the spiral inductor and the underlying substrate. This parasitic capacitance not only may increase the noise level and power consumption of the application including the spiral inductor but also may reduce the self-resonant frequency of any tuned circuit incorporating the spiral inductor.
In view of the discussion above, it is highly desirable to design an inductor has a large inductance, but occupies limited area on the surface of a semiconductor substrate. It is also desirable that this inductor cause little noise in its neighboring components.